Apparently, for dna synthesis between circular dna and linear dna, T4 phage gp43 Pol allows for dna synthesis to commence WITHOUT prior addition of a primer, whereas the primer is required for circular dna. As a comparison, a test run with DNA Pol III demonstrated that the primer is required in both cases for that enzyme. In a kinetic plot of linear dna synthesis with and without a primer, it is evident that synthesis with primer is faster, while synthesis without the primer has an initial retarded period, followed by a slower rate of synthesis.

 

Is there some kind of physiological/structural difference going from circular to linear dna that allows gp-43 Pol. to synthesize dna without a primer?

Circular DNA is more resistant than linear DNA to DNAase action or digestion. Can you check if DNA digestion or degradation takes place in your kinetic assay? Some pieces of this DNA may act as primers.